A blog of Bridge Environment, updated weekly on Thursdays, travel permitting.
Bridge Environment seeks to catalyze a cultural shift in how our society addresses environmental issues. We provide relevant and unbiased advice to any interested party, and also work to educate scientists, policy makers, and the public on how to have a more informative dialog over environmental issues.

Thursday, December 13, 2012

Fishing, a window into marine ecology

Many of us look
at nature, particularly at a healthy ecosystem, and see chaotic color, a messy
morass, a lawless lot of life forms. The science of ecology shows us otherwise.
We find collections of animals and plants within specific ecosystems, which
themselves are subdivided into habitats with distinctive characteristics.

One of the
things I like about fishing is that it’s a chance to witness the local ecology.
I started fishing as a little kid, mostly with my maternal grandfather. Ben
Harris was a third-generation San Franciscan, born in 1898 and witness to
massive environmental and economic change in the areas surrounding his and my hometown.
He used to take me fishing for sunfish in man-made lakes, which was mostly what
was left of fishing opportunities in the area by my lifetime. In high school, I
had a serious love interest whose parents owned land in the Trinity Alps of northern
California. She, her dad (named Sherwood, who better to explore a forest with?),
and I used to go fly fishing for trout on the stream that flowed through that
small plot of land. This was a vastly different experience than catching
sunfish in a man-made lake: the fish were big enough to eat (and tasty at that)
and the fishing technique required hiking and casting skills. The biggest
difference, though, was the omnipresence of ecology in our fishing strategy. We
chose flies to match naturally occurring insects in that place and season so that
they would be recognizable by the trout as common food. We identified likely
trout pools based on their suitability to shelter and feed a big fish. We aimed
our casts upstream from these pools, choosing a placement that would allow the
fly to drift naturally into them. To catch our trout, we had to be in tune with
their ecology.

Bolivar Cay in the Seaflower Biosphere Reserve, Colombia

As one of my favorite research
projects, my colleagues and I advised managers based on similar ecological
patterns. In 2000, I had the great luck to lead two expeditions to the future Seaflower
Biosphere Reserve in the Caribbean waters of Colombia. The local government
agency wanted advice on how to design the Reserve, including no-fishing and
other restricted access zones, and relied on me because of my expertise in
marine protected area design. They already had some sophisticated maps of coral
reef distributions, but wanted to consider fish communities for the sake of healthy
fisheries and vibrant diving sites. Our expeditions consisted of collecting
information on benthic communities, including coral, sponges, algae, and other
bottom dwellers; and fishing assemblages. We then used statistics to categorize
sites, grouping ones with similar species assemblages and separating those with
differences.

Not
surprisingly, we found that the benthic communities fell into categories that
varied depending on depth, proximity to land, and whether the reef was on the
exposed or protected side of the island. These categories matched what my team
had expected prior to the surveys, based on our understanding of Caribbean
coral reef ecology. What was more surprising were the fish assemblages. Despite
the mobility of fish and the potentially disruptive effects of fishing, the
fish assemblages grouped exactly the way the benthic communities had. You can
read more about our work here. Our work
confirmed the ecological basis of our definitions of habitat types, and suggested
that the Colombian government could protect a representative portion of their
coral reef ecosystems if they included similar percentages of each habitat type
in their protected areas. We were also able to reassure the government that
sites of the same habitat type would have similar value, allowing them to work
flexibly with the local fishing community to devise the most acceptable version
of fishery closures.

Dolphinfish, Coryphaena hippurus

These patterns can
also be viewed through fishing activity itself. I worked with a graduate
student, Kristin Kleisner, on a project to look at the ecology of pelagic
fishes—those that live up in the water column. We examined oceanographic
conditions using satellite data and fish abundance using catches as recorded by
observers, who took down details of fishing strategies and catches (including
discards). With these two data sets, we found that dolphinfish (Coryphaena hippurus) showed distinct
patterns. Inshore, they were more likely to be caught in shallower water and
near fronts—places where warm and cold water come into contact. Offshore, they
were more likely to be caught in deep water and farther from fronts. Once
again, ecology shined through in patterns of fish catches, and such patterns
are integral to the strategies of fishing captains who target pelagic fishes.

These patterns
play an important role in the management of fisheries through the challenge of
estimating fish abundance. Ideally we would have regular, independent,
scientific surveys for this purpose that sampled a wide variety of conditions
in a controlled fashion. However, most often we rely on catch per unit effort
(CPUE) from the fishing industry. If a fish stock is abundant, we can assume a
fishing operation will catch more fish per unit effort (e.g., hook-hour,
trap-day). We regularly assume that CPUE is a good, linear indicator of
abundance. In other words, if CPUE doubles, we assume the fish stock has become
twice as abundant. If it drops to half its original value, we assume the same
of the stock abundance. We have major challenges with using CPUE as an
indicator of fish abundance. Fishing activity is not random or consistent. In
fact, we expect smart fishermen and women to adapt their strategies to maintain
high CPUE, even if fish abundance drops. In order to see through these changing
strategies, we need information about fishing behavior so we can account for
changes, a process known as standardization by tracking different fishing
strategies and looking for changing CPUE within each strategy. Often we lack
sufficient information to do so. This problem becomes even more complex in
fisheries that target multiple species. In these fisheries, which are the norm
rather than the exception, changes in effort can also involve a shift in the
target species. When this happens, we may see dramatic drops in the CPUE for
one species simply because another becomes a more profitable target. If we have
a better understanding of the underlying ecology and how it gets represented in
catches, we can do a better job of standardizing effort and understanding and
accounting for targeting.

Recently, I
tried out a new idea for handling fishing effort. When I was in Colombia a
month ago, we examined data from many fishing trips that all used hooks on
fishing line. Some of these were used at the surface, others in midwater, and
still others deep down. Trips were categorized as surface, midwater, deep, or
some combination thereof. We had a hard time standardizing effort based on
these reports because most trips were a mix of strategies without any
indication of which were the main emphasis. We did have records of fish caught
on the trip, and I thought it might be interesting to see if we could see some
ecology reflected in the data. Even with messy data from fishing trips that
ranged across a large area and varied in time from hours to days, the ecology
did shine through. Certain trips tended to catch deep-dwelling species while
others tended to catch certain species of shallower water fish. We are now
working on an index of how heavily each trip should be weighed when calculating
CPUE, with heavier weights for trips that caught a collection of species that
were ecologically similar to the one of interest. Fishing does indeed offer a
window into marine ecology, and we can do a better job of understanding
fisheries by paying attention to the view in that window.